THE INFLUENCE OF AN ELECTRIC LIGHT ON THE CAPTURE OF OCEANIC CEPHALOPODS BY A MIDWATER TRAWL

A total of 57 comparative hauls using a rectangular midwater trawl wit h a fishing mouth area of 50m(2) (RMT 50) were carried out along the s ides of an imaginary triangle south of Madeira in 1986. A total of 125 8 cephalopods were caught, giving a mean of 22 per haul with a range f rom 0 to 67. The nets were used with a diver's light on the top bar wh ich was either switched off or was operated with a 20, 70 or 150 W bul b, powered by a car battery. A significantly greater number of individ uals per haul was caught with lights on than without lights, increasin g from a mean of 13.5-25.1, a factor of 1.8. Similarly, the number of species caught was increased from a mean of 7 to 10.4, a factor of 1.5 and the volume of cephalopods was increased from a mean of 41.1-162.3 ml, a factor of 3.9. Similar comparisons made for catches during day or night separately and on the three courses separately also showed ma rked increases with the lights. Samples show that increase in power of the lights increased the total number of cephalopod individuals caugh t. In the 12 species with more than ten individuals, in 33 of the 36 c omparisons (of number of individuals, species and volumes) there is an increase with the light. The most influenced species was Taonius pave which increased in numbers by a mean factor of 3 9 times with 20W, 4 0 times with 70W and 6.1 times with 150 W when compared with the numbe rs caught with no light. Increases by Helicocranchia pfefferi, Pyroteu this margaritifera and Pterygioteuthis gemmata are significant for som e lights. Abraliopsis pfefferi and Liocranchia reinhardti increased in lengths of individuals with increasing wattage of the lights. The res ults show conclusively that midwater trawling could be made much more cost- and time-effective by using inexpensive lights on the nets and b iological studies of so-called rare species could be greatly facilitat ed.